As is well known, the compressor case of a gas turbine engine powering aircraft is subjected to severe pressure and temperature loadings throughout the engine operating envelope and care must be taken to assure that the components remain concentric maintaining relatively close running clearances so as to avoid inadvertent rubs. Inasmuch as the engine case is thin relative to the rotor and stator components in the compressor section, it responds more rapidly to temperature changes than do other components. This is particularly true during periods of transient engine performance. Typical of these transients are throttle chops, throttle bursts, bodies and the like. Obviously it is customary to provide sufficient clearances during these transients to assure that the rotating parts do not interfere with the stationary parts.
In certain installations that utilize the axial split case it was necessary to utilize a full hoop case for the highest stages of a multiple stage compressor in order to attain adequate roundness and concentricity to achieve desired clearance between the rotating and non-rotating parts. Since the stator components, i.e., stator vanes and outer air seals are segmented, the problem was to assure that the compressor maintained its surge margin notwithstanding the fact that the outer case would undergo large deflection at acceleration and deceleration modes of operation. The cavity that exists between the outer case and the inner case formed by the segmented stator components, being subjected to pressures occasioned by the flow of engine air through the various leakage paths, presented a unique problem. In the event of a surge, the pressure in the gas path would be reduced significantly. Because the air in the cavity is captured and cannot be immediately relieved, it would create an enormous pressure difference across the stator components, cause them to distort, with a consequential rubbing of the compressor blades, and a possible breakage.
In order to withstand this pressure loading and yet achieve the roundness and clearance control of the stationary and rotating components it was necessary to incorporate a mechanism that would tie the outer case to the segmented stator components.
Moreover, since the outer case, due to temperature changes grows and shrinks faster than the rotor, the full hoop tied case as described, presented an even severer problem than would otherwise be encountered imposing on the engine designer a difficult task to assure that the clearance between the tips of the compressor blades and its outer air seal is adequate without imparting a penalty to the overall engine's performance.
We have found that we can improve on the control of the gap clearances by providing quasi-controlled thermal environment for the outer case. By virtue of this invention, the outer case is thermally isolated such that the temperature of the case is at a higher value than it would otherwise be which inherently increases the radial growth of the stator components during engine operation. The thermal isolation also slows the casing response to main gas path transient temperature changes. These aspects, together with the judicious selection of the proper coefficient of thermal expansion of the material used in the engine casing enhances the growth match-up of the stator and the rotor resulting in reduced radial clearances during part power engine operation which in turn improves compressor stability margins and engine operating efficiencies.